Conventionally, workpiece conveying systems using a robot have been proposed. For example, Patent Document 1 discloses a workpiece conveying system in which a chuck device is attached to a robot. In Patent Document 1, the chuck device has a pair of jaws (or dogs) which hold a cylindrical workpiece by workpiece contact surfaces where a V-shaped dent is formed, respectively. Note that some chuck devices may have three jaws with flat workpiece contact surfaces.